1. Field of the Invention
The present invention relates to a method and a device for producing 4-fluoroethylene carbonate. More particularly, the present invention pertains to a method of producing 4-fluoro-1,3-dioxolan-2-one, in which ethylene carbonate ((CH2O)2CO, hereinafter referred to as “EC”) directly reacts with a mixture gas of fluorine and nitrogen gases (hereinafter, referred to as “F2/N2 mixture gas”) without using a solvent, and a device used in the method.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 37 CFR 1.98.
4-fluoro-1,3-dioxolan-2-one has the common name 4-fluoroethylene carbonate (hereinafter, referred to as “FEC”).
An insoluble organic electrolyte, in which F2-containing inorganic lithium salts are dissolved in an organic solvent, is mainly used as an electrolyte for lithium ion secondary batteries. With respect to this, the electrolyte must be highly conductive, and electrically and chemically stable, and have low reactivity to a vessel and electrode material. Typically, one organic solvent cannot have all of the above various properties, thus a mixture of the organic solvents is used. Furthermore, it is known that performance of the lithium ion secondary battery is improved by adding an F2-containing organic compound to the electrolyte mixture, and fluoroethylene carbonates are frequently used as the F2-containing organic compound.
Fluoroethylene carbonate is classified into mono-fluoroethylene carbonate, di-fluoroethylene carbonate, tri-fluoroethylene carbonate, tetra-fluoroethylene carbonate, and isomers thereof according to the number of substituted fluorine atoms. Of them, FEC is known as the best additive capable of improving performance of the organic solvent for the electrolyte of the lithium ion secondary battery. If FEC is added as the additive to the electrolyte, reduction of performance of the battery caused by decomposition of the electrolyte is insignificant during an initial electrification, and explosions intermittently occurring in the lithium ion secondary battery are suppressed because the thermal stability of the battery is significantly improved. Due to these functions, the use of FEC is gradually growing.
However, in order to assure performance and safety of the battery, the quality of FEC, which is capable of being used as the electrolyte of the lithium secondary battery, is very strictly regulated so that purity is 99.8 wt % or more, moisture content is 20 ppm or less, APHA color is 50 or less, metal ion content is 1 ppm or less, and acidity (based on HF) is 50 ppm or less.
With respect to the production of FEC, a method of directly reacting EC with F2/N2 mixture gas to fluorinate EC is known as the best method in view of yield and purity. If EC directly comes into contact with F2 gas and they thus react with each other, reactivity is very high, causing local explosions, resulting in deterioration of EC. Accordingly, various modified methods, such as the use of the F2/N2 mixture gas and the use of fluoride, having low reactivity to F2, as a reaction solvent, are adopted so as to assure stability of the reaction.
U.S. Pat. No. 6,010,806 (from the year 2000) discloses a method of producing tri-fluoroethylene carbonate, in which dimethyl carbonate reacts with 3,3,3-trifluoro-1,2-propylene oxide in the presence of sodium carbonate (NaHCO3).
In the above method, costly fluoro-organics (3,3,3-trifluoro-1,2-propylene oxide) are employed as a raw material, and a reaction time is 8-40 hours, which is somewhat long. Additionally, an agitation unit is used in a cylindrical reactor, and rinsing is conducted using water to produce a final product. Furthermore, complicated purification processes, such as extraction, drying, filtration, and crystallization of organics, are conducted, and yield is no more than 57%. Since the complicated purification processes are repeated, the possibility of inflow of impurities is increased. Accordingly, it is difficult to assure a desirable quality electrolyte for the lithium ion secondary battery (in which a metal component content is 1 ppm or less, a moisture content is 20 ppm or less, and acidity is 50 ppm or less), thus the above method may not be applied to a commercial process.
WO 98/15024 discloses a method of synthesizing FEC through a fluorine substitution reaction using 4-chloro-1,3-dioxolan-2-one and KF. In this method, the halogen substitution reaction requires a high temperature and a long reaction time, selectivity is low, and a post-process, such as a rinsing process employing water or filtration, is inevitably conducted to remove used KF, and KCl and HCl byproducts. It is very difficult to produce highly pure FEC through the above complicated purification process.
Japanese Patent Laid-Open Publication No. 2000-309583 discloses a method of producing FEC, in which EC is used as a raw material in a reactor having a thermostatic bath at 50° C. and an agitator in the bath. Anhydrous hydrogen fluoride or perfluoro carbon is used as a reaction solvent, 30% F2/N2 mixture gas is fed at a flow rate of 350 ml/min, a reaction is conducted for about 40 hours while agitation is implemented at 800 rpm, and rinsing is conducted using water to remove HF. The rinsing is conducted again using 10% NaHCO3 aqueous solution, and extraction is conducted six times using 500 ml of dichloromethane. After drying is conducted using MgSO4, dichloromethane is removed, and distillation and crystallization processes are carried out. In the method, when 1.8 mol F2 gas are used, the FEC content of reactants is 64-67%, which means that 33-35 mol % of supplied F2 gas is consumed in the reaction and 3 mol F2 gas are needed to produce 1 mol FEC. Since 33% of supplied F2 gas is consumed in the reaction, the great amount of FEC is dissolved in water during the rinsing and extraction processes, thus the yield is rapidly reduced. Furthermore, vinylene carbonate is generated as a byproduct through dehydro fluorination of FEC due to moisture, thus purification may be impossible. As well, undesirably, anhydrous hydrofluoric acid is used in an amount of 5-50% of an EC weight ratio to control reaction heat, and costly F2 gas is used in an excessive amount of 1.8 mol based on EC. In the above reaction, FEC having a purity of 90% is produced in an amount of 480 g (70% yield). Re-crystallization is repeated a few times at 15° C. to produce 390 g of FEC having a purity of 99% or more. At this time, yield is 38% based on the amount of F2 used.
Japanese Patent Laid-Open Publication No. 2000-344763 discloses a method of producing highly pure difluoro ethylene carbonate, in which 30% F2/N2 mixture gas is added at a flow rate of 250 ml/min to EC in a reaction device that is the same as that of Japanese Patent Laid-Open Publication No. 2000-309583. Reaction is conducted for about 11 hours to produce difluoroethylene carbonate (hereinafter, referred to as “DFEC”) which contains 70% trans, 14% cis, 6% gem, and 9% FEC. After HF is removed by distillation, rinsing is conducted using 10% NaHCO3 aqueous solution, and extraction is conducted six times using 250 ml of dichloromethane. Thereafter, drying is conducted using MgSO4, and DFEC is separated by vacuum distillation at 5-20 mmHg. In this reaction, the amount of F2 used is 2.25 mol based on 1 mol EC, and purity is 99% or more after re-crystallization.
In Japanese Patent No. 2004-63432, 3 mol F2 gas are needed to produce 1 mol FEC. This patent is problematic in that since water is used to remove HF, problems with respect to reduced yield as described above occur.
WO 2004/076439 (Application No. PCT/EP 2004/001345) discloses a method of producing FEC, in which FEC is mixed with EC in an amount of 3-20 wt % based on a weight ratio of EC and is then reacted with 5 v/v % F2/N2 mixture gas at a low temperature of about 15-45° C. so as to control the temperature of an intense reaction between EC and fluorine gas or F2/N2 mixture gas. Neutralization is conducted using KHCO3 after the reaction is finished. After filtration, the filtrate is washed with acetone, and the acetone is removed by distillation. In this method, the time required to produce FEC is 4-5 times as long as that of the present invention, and distillation is conducted after a neutralization process using potassium hydrogen bicarbonate (KHCO3), a suspension filtration process, and a washing process using acetone. Therefore, a production process is complicated, the possibility of inflow of impurities is high, and it is commercially nonviable.
In Japanese Patent Laid-Open Publication Nos. 2000-309583 and 2000-344763, EC is dissolved in an anhydrous hydrogen fluoride solution so that an EC content is 5-50 wt %, and a F2/N2 mixture gas is supplied and then reacted. However, these patents are disadvantageous in that since HF is generated in 20 wt % of the amount of FEC only through the reaction between EC and the F2/N2 mixture gas, it is not easy to reduce the amount of HF to a set value or less after the reaction is finished. Accordingly, it is difficult to apply these patents to a commercial process and they are economically inefficient. Furthermore, it is difficult to reduce the acidity, which is the most important property of an electrolyte, to a set value or less even when a re-crystallization process is implemented.